The present invention relates to splicing apparatus and methods, particularly as applied to optical fibers.
Optical fibers are finding increasing applications in many disciplines. Perhaps most notable among these applications is communications where optical fibers are replacing electronic cables as information carriers of choice. In some applications, such as oilwell logging, extremely long, high-strength, moisture resistant fiber optical cables are required.
Optical fibers are typically drawn from a rod. This drawing approach can form very long fibers, but it is not practicable to form single fibers long enough for many oil-well logging projects. Thus, splicing technology emerges as a critical element in such applications. This is in addition to the importance of splicing technology in repairing damaged cables and renovating cable networks already in place.
In the case of a cable repair, a damaged segment can be removed, the outer layers stripped back, and the remaining ends cleaved and fused to form a splice. Generally, this splice is weaker than the original fiber, so strain relief devices are added in addition to intermediate buffer layers, and other layers specific to the cable of interest. The strain relief devices and other post-manufacture coverings tend to bulge relative to the original coverings remaining on either side of the splice. This is not particularly awkward in some repair situations, but it can be problematic where space is at a premium.
However, where a very long cable is to be originally formed from segments and delivered on a spool, bulky splices are to be avoided, especially in diameter critical applications such as oil-well logging. It is much more difficult to wind and unwind a spool with a fiber cable with significant bulges along its length. Furthermore, even despite strain relief, a new cable can be subject to considerable bending and stress during installation, so that relatively weak splice points can be a source of unreliability.
Splice locations are not only a source of mechanical weakness. They tend to be the locus of transmission losses due to core misalignment and end surface reflections. Thus, if the alignment during splicing is imperfect, a multi-segment cable can have unacceptable light loss. Even if the cable is aligned sufficiently just after splicing, minor physical damage at splice locations can cause optical performance to drop below a satisfactory level.
The problems with splices are complicated in the case of hermetically sealed and other specially coated fibers. Moisture can readily penetrate most fiber jacket and buffer materials and attack the fiber itself so as to weaken it. This weakening can be quite dramatic over an extended period of time. To prevent this attack, the fiber can be coated with a hermetic seal so that moisture penetrating the outer layers of a fiber cable cannot reach the fiber itself.
U.S. Pat. No. 4,512,629 to Hansen et al. discloses an approach to manufacturing a fiber cable in which a hermetic coating is applied to the fiber as it is being formed off a draw tower. Before the drawn fiber has a chance to cool it is exposed to methyl acetylene or other carbon-bearing gas. The pyrolysis of the methyl acetylene in a non-oxidizing environment yields a carbon film on the fiber. This glassy film serves as an effective hermetic seal.
However, if conventional splicing techniques are used, this seal is necessarily broken at the splices. When exposed to water molecules, the splices become weak links in the fiber optical chain greatly diminishing the value of the intact hermetic seal. The problem is compounded in that, in a cable with a hermetically sealed fiber, the splice is both the physically weakest point to begin with and the most vulnerable to attack by moisture.
One approach considered is to place a spliced fiber in a heated chamber containing methyl acetylene to form an amorphous carbon coating over the exposed fiber just after splicing. However, splices coated in this matter have failed to provide strength and moisture resistance comparable to that of the splice-free portions of the cable. It is believed that the coating so applied includes small chunks which are relatively susceptible to damage during cable tension and flexure, and are relatively likely to permit moisture to penetrate to the fiber.
Thus, there are several problems with existing splicing technology. These include, splice bulk, splice weakness, poor optical performance at the splice and breach of a hermetic seal at the splice. Accordingly, an object of the present invention is to provide a splicing method and apparatus for forming splices which differ minimally from the rest of the incorporating cable in diameter, in mechanical performance, in optical performance, and in resistance to attack by moisture. In particular, optical fiber cables suitable for oilwell logging are needed.